1. Field of the Invention
The present invention is generally directed to solar cells and more specifically to a solar cell having a combined metallization and the manufacturing method for producing same.
2. Description of the Related Art
High-efficiency solar cells of polycrystalline or monocrystalline semiconductor material, for example of silicon, must be optimized toward high efficiency in every detail. For example, wafers are used that have only a light doping and therefore a low hole density. The pn-junction is produced at the front or light incidence side by, for example, drive in of phosphorous, whereby the highly doped zone created in this way has only slight depth.
In order to improve the current conduction at the backside of the solar cell, the p-doping directly under the backside contact is strengthened and reinforced. Aluminum is usually used for this purpose. The aluminum is applied, for example, by vapor-deposition or by being printed onto the backside and being driven in or, respectively, alloyed in.
Various methods and processes are known for the metallization of the solar cells, i.e. for the manufacture of the current-conducting contacts. A structured metallization can be produced in a simple way by printing an electrically-conductive paste on. Arbitrary electrode structures can be produced in a silkscreening process by using a paste containing metal particles.
Metallizations can also be produced by vapor-deposition of a metal layer and can be reinforced in a further step, for example by dip-soldering, voltaic deposition or by currentless chemical deposition. However, it is problematical to produce a metallization by chemical deposition directly onto untreated semiconductor surfaces, for example by nickel deposition from chemically-reductive baths onto a basic base. However, trials for manufacturing larger item numbers exhibited too narrow a process window.
Front side contacts applied with silkscreening can only be realized in structural widths beginning at approximately 80 .mu.m. As a consequence of the high glass component in the paste, contacts that are printed even narrower exhibit a greatly diminished electrical conductivity. Finer structure, however, is required for the front side contact to enhance the performance of the solar cells by reducing the occlusion of active solar cell area caused by the contact.